Spunbond tower having electrostatic device

ABSTRACT

The invention relates to a spunbond tower that includes a die ( 103 ), a device ( 106 ) for cooling the filaments formed by the die, a device ( 107 ) for stretching the cooled filaments, a system ( 108 ) for forming a non-woven web ( 110 ) on a mat, said system including an electrostatic device ( 117 ) and a dehumidifier mounted at least on a formation air inlet between the bottom of the stretching device and the top of the formation system.

The present invention relates to spunbond towers having an electrostatic device which allows bundles of filaments to be ungrouped.

The electrostatic device operates on the principle of the Corona effect which brings about ionisation of the air near a point subjected to electrical potential.

The Corona effect requires:

-   -   a small electrode which may be either a point contact (or a         point type network) or a wire,     -   an electrical field which is produced by the difference in         potential established between the electrode and an opposite         electrode which generally comprises a conductive planar plate         which is located opposite the main electrode.

The small size of the electrode brings about a concentration of the field lines which can exceed the ionisation threshold and bring about ionisation of the air.

Depending on the polarity applied, the Corona effect is said to be positive or negative and results in different ionisation of the air (FIG. 6).

In both cases, particles having positive and negative polarity are produced and are then carried by the electrical field towards the electrode or the planar plate having opposite polarity. During their movement, those particles collide with other particles present in the volume and may recombine and cancel out their charge or create new charges. Thus, during those collisions, the filaments will also receive electrostatic charges and be subjected in turn to the electrostatic forces created by the electrical field. Since the filaments are not charged identically, they will not be subjected to identical forces and movements and will thereby be dispersed in the space between the electrode and the planar plate.

On the other hand, the filaments are preferably charged with the same polarity. The material constituting the filament is naturally electropositive or electronegative and therefore has a tendency to more readily accept charges of the polarity corresponding to its electrostatic affinity. Owing to that charge, the filaments will have a tendency to repel each other and thereby to occupy the volume of air available in a more uniform manner.

The important parameters of the device are the voltage applied between the electrodes (generally several tens of kilovolts, from 10 to 70 kilovolts) and the current which is produced (by movement of the ions) between those same elements (several tens of milliamps per metre of length, from 2 to 20 mA per metre of length).

The voltage applied directly influences the force applied to a charged particle. A particle having a charge Q is subjected to a force F=Q×E, E being the electrical field which is directly proportional to the electrical voltage.

The current obtained reflects the quantity of the charges which move between the electrodes. Thus, an increase in the current indicates an increase in the quantity of charges present in the volume between the electrodes and consequently an increase in the probability of depositing charges on the filaments and modifying their trajectory.

The major advantage of this electrostatic device is that it allows dispersal of the groupings of filaments which are generated by the equipment located upstream. Those groupings generally result from occurrences of turbulence or local instances of heterogeneity of the air flows, which it is difficult or impossible to completely dispense with.

By inducing an electrical charge in the filaments, the electrostatic device brings about their relative movement in space either by the electrical field created by the electrostatic device itself or by repulsion with the adjacent filaments which have the same polarity.

FIG. 7 shows two examples of trajectories followed by the filaments when there is no electrostatic device (FIG. 7.1) and when there is one (FIG. 7.2).

That effect on the filaments allows the appearance of the non-woven web to be greatly modified, as shown in FIG. 8.

As indicated in FIG. 8.1, without any electrostatic device the web generally has a cloudy appearance which comprises zones containing a large number of filaments which are substantially interleaved and zones containing a much smaller number of filaments. All the physical properties of the web (such as the basis weight, behaviour under a traction load, permeability to a gas, a liquid or a powder) are affected by that heterogeneity.

When there is an electrostatic device, the zones containing more filaments are much more diffuse, their size increases, they overlap each other (FIG. 8.2). Consequently, the web gains uniformity and all the physical properties sought by users are improved.

However, it is found that, within a few hours, the device for producing a non-woven web no longer provides a satisfactory web such as the one in FIG. 8.2, but instead provides a web such as the one in FIG. 8.1.

The invention overcomes this disadvantage and allows a web having good properties to be obtained for a long operating time.

The invention relates to a spunbond tower successively comprising in a downward direction:

-   -   a die forming filaments from plastics material,     -   a device for cooling the filaments formed by the die,     -   a device for drawing the filaments cooled by the cooling device         and     -   a system for forming, on a conveyor belt, a non-woven web from         the filaments drawn by the drawing device, that system         comprising an electrostatic device and a forming air inlet being         provided between the bottom of the drawing device and the top of         the forming system, characterised by a first dehumidifier for         the forming air.

The Corona effect explained above in a very simplified manner is in fact an extremely complex physical phenomenon. The molecules and ions created in that reaction are very greatly dependent on the polarity and the composition and the nature of the gas present in the space between the electrode and the planar plate.

Thus, chemical molecules present in the gas will be able, in the same manner as the filaments, to receive an electrostatic charge and be subjected to the effects of the electrostatic forces. Those molecules will be subjected to a horizontal movement either towards the electrode or towards the planar plate and will ultimately be able to become deposited on those elements, causing contamination of the device.

The confirmed effect of that contamination is a reduction in the Corona effect which brings about a reduction in the current having constant voltage and which causes an increase in the voltage necessary to establish a given current.

This has two effects which disrupt the system:

-   -   the first is that, by reducing the current, the quantity of         charges present in the space is reduced and therefore the         probability of charging the filaments. This results in a         reduction in the charges present on the filaments and         consequently a reduction in the quantity of filaments which         move;     -   the second is that, by increasing the voltage, the mean         electrical field which exists between the electrode and the         planar plate is increased. In that manner, with an equivalent         charge, the filaments are subjected to a greater load and         therefore to a greater magnitude of movement. During their         movement, those filaments come into contact with the walls of         the channel which has the effect of creating faults in the         non-woven fabric.

Generally, during the use of the tower, the effect of clogging is characterised by an increase in the voltage applied in order to maintain the current at the desired value then, when the maximum voltage available is reached, by a reduction in the current.

In parallel with this reduction in current, the appearance of the web changes progressively from the quasi-uniform appearance of FIG. 8.2 into the heterogeneous appearance of FIG. 8.1. Below a given value of current, the appearance becomes too cloudy and the product is no longer acceptable to the end user. The tower is stopped in order to allow the device to be cleaned. The criterion for judging whether to decide to stop for cleaning is generally a minimum current level below which the non-woven fabric is considered to be non-compliant.

Dehumidification allows efficiency losses over time owing to contamination to be greatly limited.

It is also preferable to mount a second dehumidifier on the (air inlet) apertures of the drawing device. Since the drawing air is injected under pressure, the reduction in pressure cools the air and, owing to condensation, creates water droplets which are very detrimental to the electrostatic device. The time for which the tower operates well is thereby increased by dehumidification.

The cooling device preferably has an air inlet and a third dehumidifier is mounted on the cooling air inlet. The volume of air dehumidified is thus relatively large and little contamination remains in the tower.

The tower preferably comprises a fourth dehumidifier which is mounted on an air inlet aperture in the forming system.

According to an embodiment allowing the electrostatic device of the tower to be used as an initial electrostatic dust separator, the tower has an intake device below the conveyor belt and the first and/or second and/or third and/or fourth dehumidifier comprises a device for recycling at least a portion of the forming air and/or cooling air and/or air being introduced via the air inlet aperture and/or the drawing air which is drawn in by the intake device at the forming air inlet and/or at the cooling air inlet and/or at the inlet aperture of the forming system and/or at the drawing air inlet. The tower may have a humidity sensor at an air inlet in the tower, controlling, by means of a control device, an adjustment register of the air flow drawn in by the intake device. The tower may also have a humidity sensor for air which is introduced into the tower controlling, by means of a control device, a register adjusting the flow of air recycled.

The tower may have below the conveyor two separate intake devices, one called a formation chamber intake device below the forming system and the other called a maintenance chamber intake device downstream of the formation chamber and there is a device for recycling the air of the maintenance chamber at the spunbond tower. It is possible to recycle all the air. Preferably 20% to 50% by volume is recycled which corresponds to the air entering at the top of the forming device.

According to one of the simplest embodiments, the dehumidifiers comprise, in a downstream direction in the direction in which the air is introduced into the tower, a heat exchanger for cooling the air in order to condense the humidity therein to form water droplets, a droplet separator and a reheater in order preferably to bring the humidity rate of the dehumidified air to a value between 20% and 30%.

The relative humidity of air is the relationship expressed as a percentage of the partial pressure of water vapour contained in the air relative to the partial pressure of saturated vapour under identical temperature and pressure conditions. The relative humidity of the air can be measured using relative humidity sensors which directly convert the humidity level of the air into an electrical signal.

In the appended drawings which are given purely by way of example:

FIG. 1 illustrates a spunbond tower according to the invention,

FIGS. 2 to 4 illustrate variants of a spunbond tower according to the invention,

FIG. 5 is a graph showing the voltage and the current, ordered as a function of the time in days, of the electrostatic device of the spunbond tower according to the invention,

FIG. 6 is a diagram illustrating the phenomena which occur in an electrostatic device,

FIG. 7.1 is a diagram illustrating the distribution of the filaments when there is no electrostatic device, whilst there is one in FIG. 7.2,

FIGS. 8.1 and 8.2 are views of the non-woven fabric obtained in accordance with the diagrams of FIGS. 7.1 and 7.2, respectively.

In the method for producing non-woven webs by spunbond technology, as set out succinctly in FIG. 1, the polymer in the form of granules is molten in an extruder then drawn through a die (103) in the form of continuous filaments (104). The fumes emitted during the drawing are collected by an acquisition device (105). The filaments are then cooled in a cooling device (106) by a current of air at a controlled temperature and speed, then introduced into a drawing device (107). That device allows a tension force to be applied to the filaments which allows the molecular chains to be orientated and the desired diameter to be obtained.

At the outlet of the drawing device, an additional device called a forming system (108) is generally provided to allow the filaments to be deposited on a conveyor belt in order to form the non-woven sheet (110). The main function of that forming device is to reduce the speed of the filaments, to disperse the packets of filaments over the width of the machine in a manner which is as uniform as possible and to allow random and homogeneous deposit on the conveyor. The device (107) and the device (108) form a device for moving the filaments downwards by means of a current of air.

An intake device (111) located below the web of the conveyor allows the sheet to be pressed and maintained on the conveyor. The non-woven sheet subsequently passes through a compacting device (112) and a consolidation device (113). The latter may be a calendering system or any other consolidation device (mechanical needling, chemical bonding, bonding by fluid jet). The sheet is conveyed towards the remaining steps of the method (processing, winding).

The drawing device, which is installed vertically, is constituted by a continuous aperture (201), in which the curtain of filaments is introduced.

The effect of drawing is generally obtained by a current of air which flows in a downward direction and which carries the filaments by friction with the air. The current of air may be either generated by the flow of air introduced for cooling the filaments (closed system) or by injection of additional air into the drawing device, which brings about a general flow owing to the Venturi effect (open system).

At the outlet of the drawing device, the forming system generally comprises an aeraulic system (for example a diffuser, FIGS. 1-114) which modifies the flow profile of the air at the outlet of the drawing device. It is particularly advantageous to provide in a judicious manner additional apertures for introducing forming air (FIGS. 1-115 and FIGS. 1-116) which allow control of the flows and prevention of the appearance or untimely development of turbulence.

A device of the electrostatic type (FIGS. 1-117) judiciously complements the effectiveness of the diffuser with regard to the ungrouping of the filament packets.

It will be noted that, if the relative humidity of the air being introduced is brought below 50% and preferably to values between 20% and 30% by weight, the loss of efficiency disappears almost completely and the device maintains constant behaviour over several days of production, and even several weeks. The reduction of the relative humidity of the air to a value less than 20% does not bring any significant additional improvement.

Since the relative humidity of the air desired is generally less than the ambient conditions encountered in production plants, the solution adopted for achieving the relative humidity of the air required is to cool the air below the dew-point in order to condense the excess humidity, followed by reheating which allows the desired temperature to be reached again.

A device of the type indicated in FIG. 1 is provided in the air for cooling the filaments and comprises an air/water exchanger for cooling (1001), a droplet separator (1002) provided with a condensate outlet hole (1003). A temperature sensor (1004) located downstream of the droplet separator allows control and adjustment of the temperature at the outlet of the cooler, acting either on the water flow or on the temperature of the water in the cooler. By being cooled, the air is thus brought to the dew-point temperature desired for the method. The value sought is generally between 5° C. and 15° C. and preferably less than 10° C. The desire for lower values necessitates devices which consume more energy and do not provide a sufficiently great improvement to justify the operating costs that are necessarily higher. Subsequently, a reheater (1005) allows air to be brought to the final temperature required, generally between 10° C. and 35° C., more usually in the range from 15° C. to 30° C. The power of the reheater is adjusted by means of a temperature/humidity sensor (1006) which is located downstream of the reheater. By the humidity being measured, the user can thereby control the relative humidity obtained. The device can also be improved by automatically controlling the temperature of the air at the outlet from the cooling operation in accordance with the relative humidity finally sought.

An identical device is provided in the air injection inlet of the drawing device and comprises the cooler (1007), the droplet separator (1008) with a condensate outlet hole (1009) and the reheater (1011). The temperature at the outlet of the cooler is controlled by means of the temperature sensor (1010). The final temperature and humidity are controlled by means of the temperature and humidity sensor (1012).

Controlling the humidity of the air in the region of the injection apertures of the diffuser is particularly essential and all the more important because the air introduced via those apertures passes near the electrodes. Air (humidity) can be processed by a device which is identical to the preceding device, that is to say, cooling, elimination of the condensate and reheating. That device may optionally be avoided when the flow of air discharged by the intake device (FIG. 1 111) located below the web being formed only discharges a quantity of air corresponding to the air injected into the apertures of the drawing device and the air introduced at the inlet of the drawing device.

Thus, as illustrated in FIG. 2, the total flow leaving the forming system (Q4) is constituted by the flow carried by the drawing unit (Q1) supplemented by the flow carried by the air introduction apertures of the forming system (Q2 and Q3). The proportion between the flows may vary in accordance with the geometry of the diffuser and the apertures. In general, the flow Q1 carried by the drawing device represents from 50% to 80% of the total flow Q4 leaving the forming system, the flow Q2+Q3 being introduced via the introduction apertures of the forming system being between 20% and 50% of the flow Q4.

If the flow Q5 drawn in by the intake device located below the forming conveyor is less than the flow Q4 being discharged from the forming system, a portion thereof is therefore delivered as two flows Q6 and Q7. When the forming system is provided inside a vessel (1101) which insulates the device from ambient air, the flows Q6 and Q7 are drawn in again at Q2 and Q3 in the region of the apertures of the forming system. An opening formed in the insulating vessel allows the flow Q8 necessary for balancing the entirety of the flows to be introduced or delivered.

When the installation is in operation, the temperature injected in the region of the flows Q2 and Q3 progressively increases, bringing about a reduction in the relative humidity. After a few minutes of operation, the assembly becomes stabilised at the value sought (temperature and humidity).

A sensor (1102) located in the intake zone of the flows Q2 and Q3 allows measurement of the temperature and humidity values. It can be connected by means of an adjustment device (1103) to a motorised register (1104) which allows control of the flow drawn-in by the fan (1105).

Other variants of the balancing device of the flows can also be provided as indicated in FIGS. 3 and 4.

FIG. 3 shows a device comprising a network of sheaths (1201) which allow a portion of the air being discharged from the intake fan to be moved towards the insulating vessel. The flow Q5 which is drawn in by the fan is equal to the flow Q4 leaving the forming system. At the delivery of the fan, the flow Q5 is divided into a flow Q6 which is discharged outwards and a flow Q7 which is recirculated towards the insulating vessel. The flow Q7 is adjusted, for example, by means of a motorised register (1203). A sensor (1202) installed in the insulating vessel allows control of the temperature and humidity of the air. The register (1203) may optionally be automatically controlled by the measurement of temperature and humidity provided by the sensor (1202) by an automatic control system.

That device allows adjustment of the proportion of flow recirculated without modifying the quantity of air drawn in via the web of the conveyor. The web is often affected by other parameters of the method and the fact of varying that value in order to control the temperature and humidity in the insulating vessel, as indicated in FIG. 2, may cause the appearance of new faults in the non-woven web.

The faults created by poor adjustment of the intake flow below the conveyor may be holes in the web, half-moons or other faults which are connected with the fact that, since the filaments are not sufficiently maintained on the web of the conveyor by the suction, they move owing to currents of air.

FIG. 4 shows a device comprising a double intake system below the conveyor. The first device (1301) which is called a formation chamber and which is located directly under the outlet of the forming device acts directly during the formation of the non-woven web on the conveyor. The second intake device (1302) which is called a maintenance chamber is located downstream in accordance with the movement of the conveyor. It ensures good maintenance of the web during transport as far as the pressing roller or the consolidation device.

The two devices are adjustable independently of each other and may each comprise a system for recirculating air. In general, the air from the formation chamber (flow Q5) is completely discharged outwards so as to eliminate in an effective manner the gas products from the Corona effect. The air from the maintenance chamber (flow Q9) is recirculated partially or completely by means of the motorised register (1304) in order to obtain the temperature and humidity values required to be measured by the sensor (1303).

By means of the control combined with the cleanliness and relative humidity of the air which passes into the electrostatic device, by means of devices such as those described above, there is obtained behaviour of the electrostatic device as illustrated in FIG. 5, that is to say, complete stability over several days of operation. 

1. Spunbond tower successively comprising in a downward direction: a die forming filaments from plastics material, a device for cooling the filaments formed by the die, a device for drawing the filaments cooled by the cooling device and having a bottom, and a system for forming, on a conveyor belt, a non-woven web from the filaments drawn by the drawing device, that system comprising a top and an electrostatic device and a forming air inlet being provided between the bottom of the drawing device and the top of the forming system, comprising a first dehumidifier for the forming air.
 2. Spunbond tower according to claim 1, comprising an air inlet into the drawing device and a second dehumidifier which is mounted on the drawing air inlet.
 3. Spunbond tower according to claim 1, in which the cooling device has a cooling air inlet, comprising a third dehumidifier which is mounted on the cooling air inlet.
 4. Spunbond tower according to claim 2, in which the cooling device has a cooling air inlet, comprising a third dehumidifier which is mounted on the cooling air inlet.
 5. Spunbond tower according to claim 1, in which an air inlet aperture is provided in the forming system, comprising a fourth dehumidifier for the air which is introduced via the aperture.
 6. Spunbond tower according to claim 2, in which an air inlet aperture is provided in the forming system, comprising a second dehumidifier for the air which is introduced via the aperture.
 7. Spunbond tower according to claim 3, in which an air inlet aperture is provided in the forming system, comprising a third dehumidifier for the air which is introduced via the aperture.
 8. Spunbond tower according to claim 1, wherein the first dehumidifier comprises, in a downstream direction in the direction in which the air is introduced into the tower, an air/water exchanger for cooling the air in order to condense the humidity therein to form water droplets, a droplet separator and a reheater in order to bring the relative humidity of the air to a value between 20% and 30%.
 9. Spunbond tower according to claim 1, which has an intake device below the conveyor belt, wherein the first dehumidifier is constituted by a device for recycling at least a portion of the forming air being introduced via the air inlet aperture the drawing air which is drawn in by the intake device at the forming air inlet.
 10. Spunbond tower according to claim 3, which has an intake device below the conveyor belt, wherein the third dehumidifier is constituted by a device for recycling at least a portion of the cooling air being introduced via the air inlet aperture the drawing air which is drawn in by the intake device at the cooling air inlet.
 11. Spunbond tower according to claim 5, which has an intake device below the conveyor belt, wherein the fourth dehumidifier is constituted by a device for recycling at least a portion of the air being introduced via the air inlet aperture the drawing air which is drawn in by the intake device at the inlet aperture of the forming system.
 12. Spunbond tower according to claim 2, which has an intake device below the conveyor belt, wherein the second dehumidifier is constituted by a device for recycling at least a portion of the air being introduced via the air inlet aperture the drawing air which is drawn in by the intake device at the drawing air inlet.
 13. Spunbond tower according to claim 9, comprising a humidity sensor at an air inlet in the tower, controlling, by means of a control device, an adjustment register of the air flow drawn in by the intake device.
 14. Spunbond tower according to claim 9, comprising a humidity sensor for air which is introduced into the tower, controlling, by means of a control device, a register adjusting the flow of air recycled.
 15. Spunbond tower according to claim 9, comprising below the conveyor two separate intake devices, one called a formation chamber intake device below the forming system and the other called a maintenance chamber intake device downstream of the formation chamber.
 16. Spunbond tower according to claim 15, characterised by a device for recycling the air of the maintenance chamber at the spunbond tower. 